1. Field of the invention
This invention relates to a semiconductor position sensitive detector. In particular, this invention relates to a semiconductor position sensitive detector which is made to have the same characteristic of photoelectric sensitivity for infra-red light as that of a usual photo-diode, and can easily be incorporated into an integrated circuit device, especially, in a bipolar integrated circuit device.
2. Description of the prior art
Japanese unexamined publication No. Hei 2-224281 shows the first prior art of this invention. In actuality, FIG. 3a is a sectional view showing the structure of this prior art semiconductor device, and FIG. 3b is the plane view of this device. As shown in FIG. 3a, this device is comprised of an n-type semiconductor substrate 110 and a p-type resistance layer 111 formed on substrate 110. This layer 111, that is, a light receiving surface, is designed to form strips on the surface of substrate 110, as shown in FIG. 3b.
The output of this device is obtained in the form of a current. Therefore, in order to improve the S/N ratio of this device for heat noise, the resistance value of this layer ill should be as high as possible. In usual, a layer having a resistance value of several hundreds K.OMEGA. is utilized as layer 111. To accomplish this resistance value without forming stripes on the substrate surface, the impurity concentration of layer 111 should be controlled extremely low, for example, about 10.sup.14 cm.sup.-3. On the other hand, as is the case of said prior art, by forming stripes on the substrate surface, this resistance value can be accomplished with the impurity concentration of 10.sup.15 cm.sup.-3.
In the device shown in FIGS. 3a and 3b, the incident position of a light is expressed by the following equation as shown in FIG. 3c, which shows the equivalent circuit of this device: EQU x'=l'X I1'/(I1'+I2'),
where l' means the length of layer 111, I1' and I2' mean the photoelectric currents detected through anode electrodes 112 and 113, respectively.
Although the device described in said publication No. 2-224281 has a PIN structure, the device shown in FIGS. 3a and 3b has a PN structure. For the purpose of finding a right position, there is no difference between the two structures. In addition, a cathode electrode 114 is provided on the top surface of this device, instead of the bottom surface.
According to said structure, the device shown in FIGS. 3a and 3b has a photoelectric sensitivity as shown in FIG. 3d. As is evident from this figure, the photoelectric sensitivity of this device does not have a constant value of the photoelectric current within the light receiving region. This is because the light receiving surface is configured to be stripes. In other words, the photoelectric sensitivity of this device varies depending on positions, like waves. As a result, the total photoelectric sensitivity of this device decreases by 20 to 40% compared with that of a simple photo diode made of the same materials.
FIGS. 4a and 4b show the structure of a second prior art of this invention. In actuality, the device of this example is structured by incorporating the device of the first prior art into a bipolar integrated circuit device. Therefore, the basic structure of this device is almost the same as that of the first prior art device, except that an n-type layer 120 is epitaxially grown on p-type substrate 126. Resistance layer 121, anode electrodes 122 and 123, and cathode electrode 124 have the same structures as those in the device shown in FIGS. 3a and 3b respectively.
Besides the disadvantages mentioned in the first prior art device, the second prior art device has the following disadvantage. That is, n-type layer 120 only has a thickness 3 to 10 .mu.m. Accordingly, in the case of infra-red light, for example, the light having 850 nm wavelength, this layer 120 having 10 .mu.m thickness can only absorb 40% (from an actual measurement) of the incident light. The rest of the incident light reaches p-type substrate 126, thus further lowering the photoelectric sensitivity.
As explained above, the prior art semiconductor devices have the following disadvantages.
(1) In order to improve the S/N ratio for heat noise, the light receiving surface of the prior art devices are configured to be a stripe shape. Therefore, the photoelectric sensitivity of the light receiving surface of these devices becomes a wave like form. The total photo-electric sensitivity decreases by 20 to 40% compared with that of a simple photo-diode made of the same materials.
(2) In the case where said devices are incorporated into a bipolar integrated circuit, only 40% of an incident light can be absorbed by the n-type layer. This is because the n-type layer is grown very thin on the substrate. The rest of the incident light passes the n-type layer and reaches the p-type substrate, thus further lowering the photoelectric sensitivity of these devices.